We need to fully involve parents in eveyrthing we do and explain the Eyfs to them so they understand the processes we go through to give them the best learning and development experience.
You would be pretty stuffed if a parent refused you permission for obs, so you need to keep it very positive... without obs you cannot plan, without planning you cannot give the child the best. They are what you would do anyway to be the best practioner you possibly can be... so now with Eyfs you are taking it a stage further and writing some of them down.
Also mention that they do not detract from your time with the child because you are doing them as you play... and a photo speaks volumes so you need permission for those with specific permission for what you are allowed to do with them.
Obs help you to determine children's learning styles, spot schemas in their play, give you a basis from which to work with the child.
Obviously you want parents to contribute to obs and this is a very important part of your discussions with them. You need to know what the child is doing elsewhere etc... this helps you meet Principle 2.2 of the eyfs as well.
Hope something here helps!